---
title: "Converting IDMT scheme to differential protection"
subject: "Switchgear and Protection"
chapter: 7
key: "semester_6/switchgear_and_protection:idmt-to-differential-transformer"
source: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/idmt-to-differential-transformer
site: BCT Engineering
---

# Converting IDMT scheme to differential protection

Switchgear and Protection · Chapter 7: Protective Relays

- Asked in: 2074 Bhadra, 8 marks
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/idmt-to-differential-transformer
- Whole chapter: https://www.bctengineer.com/md/semester_6/switchgear_and_protection/chapter-7

## Question

Describe how an overcurrent and earth-fault protection scheme by IDMT relays for a transformer could be converted into the form of differential protection. What are the advantages over the other protection schemes as applied to transformer?

## Short answer

An IDMT overcurrent scheme can be converted to differential by connecting CTs on both sides through pilots and placing relays in the difference path. Biased relays with harmonic restraint correct for phase shift and inrush. Advantages over IDMT: instantaneous tripping, absolute selectivity, high sensitivity, no need for grading.

## Answer

An IDMT overcurrent and earth-fault scheme on a transformer uses CTs and relays on each side separately. By connecting the same CTs (on both sides) together through pilots and placing the relay in the difference path, the scheme becomes a **differential (Merz-Price) scheme**.

#### Conventional IDMT O/C and E/F scheme

```text
HV CTs ─► 2 × O/C relays + 1 × E/F relay (residual)
LV CTs ─► 2 × O/C relays + 1 × E/F relay (residual)
Each relay sees only its own side current
```

These relays are graded with downstream relays, so they are slow (0.5–2 s) and cannot tell an internal fault from an external one.

#### Steps to convert into differential protection

1. **Use CTs on both sides** of the transformer and choose their ratios so that the secondary currents are equal at full load (CT ratios in inverse proportion to the voltage ratio).
2. **Correct the phase shift:** connect the CTs on the star winding in delta and the CTs on the delta winding in star (for a Δ/Y transformer). This also removes zero-sequence current, so external earth faults do not cause tripping.
3. **Join the CT secondaries by pilot wires** so that currents circulate between the two CT groups.
4. **Move the relays** from the CT secondaries into the spill (difference) path between the pilots, one relay per phase.
5. **Replace plain O/C elements with biased (percentage) relays** that have operating and restraining coils, and add **harmonic restraint** against magnetising inrush.
6. **Earth-fault element:** a **restricted earth-fault (REF)** relay can be formed from the three line CTs and a neutral CT of the star winding.

```text
CT(Y-conn)─┐  pilot   ┌─CT(Δ-conn)
           ├──[87]────┤
           │  spill   │
HV ═══[ TRANSFORMER Δ/Y ]═══ LV
(HV delta winding: CTs in star; LV star winding: CTs in delta)
```

#### Advantages over other schemes

| Point | Differential | IDMT O/C & E/F |
|---|---|---|
| Speed | Instantaneous (1–2 cycles) | Time-delayed, graded |
| Selectivity | Absolute: only internal faults | Relative: needs grading |
| Sensitivity | High; detects low-level winding faults | Must be set above load |
| Load current | No effect | Limits setting |
| Coordination | Not needed with other relays | Needed |
| Damage | Minimum, fast clearance | More damage, longer arc |

Other advantages: the zone is clearly defined by the CT positions; it protects the windings and the bushings/connections between the CTs; and it works at any loading level. The O/C and E/F relays are then kept only as back-up.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. To convert O/C and E/F IDMT protection to differential protection, the relay is moved:
   A. Into the primary circuit
   B. Into the spill (difference) path between pilots joining the two CT groups
   C. To the busbar
   D. Into the neutral earth
   Answer: B. CT secondaries are joined by pilots and the relay carries only the difference. (Key point: Conversion concept)

2. CT ratios on the two sides are chosen so that:
   A. Secondary currents are equal at full load (inverse of voltage ratio)
   B. Primary currents equal
   C. Both are 5 A rated
   D. Ratio equals turns ratio
   Answer: A. Equal secondaries give zero spill under normal load. (Key point: CT ratio choice)

3. Which additions make the converted scheme reliable?
   A. Time grading
   B. Biased restraining coils and harmonic restraint
   C. Open pilots
   D. Larger fuses
   Answer: B. Bias handles mismatch and harmonic restraint handles inrush. (Key point: Added features)

4. A key advantage of differential over IDMT O/C and E/F is:
   A. Needs grading
   B. Instantaneous, absolutely selective and more sensitive to low-level winding faults
   C. Load limits its setting
   D. Provides back-up
   Answer: B. It operates in 1-2 cycles only for internal faults and is independent of load. (Key point: Advantage)

## Related in Switchgear and Protection

- [IDMT relays across 132/11 kV transformer (3500 A)](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Aidmt-132-11kv-3500a) — Switchgear and Protection, chapter 7
- [IDMT relays across 132/33 kV transformer (2000 A)](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Aidmt-132-33kv-2000a) — Switchgear and Protection, chapter 7
- [IDMT relays across 132/33 kV transformer (2700 A)](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Aidmt-132-33kv-2700a-graph) — Switchgear and Protection, chapter 7
- [Combined phase and earth fault protection of generator](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Acombined-phase-earth-fault-generator) — Switchgear and Protection, chapter 7
- [Design IDMT grading for relays A and B](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Aradial-feeder-idmt-design-ab) — Switchgear and Protection, chapter 7
- [Differential protection of power transformer](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Atransformer-differential-protection) — Switchgear and Protection, chapter 7

## Related in other subjects

- [Restricted earth fault protection](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Arestricted-earth-fault) — Power Plant Design, chapter 4
- [Unit generator-transformer schemes](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Aunit-generator-transformer-schemes) — Power Plant Design, chapter 4, asked 3×
- [Advantages of electrical energy over other forms](https://www.bctengineer.com/question.md?key=semester_7%2Futilization_of_electrical_energy%3Aadvantages-electrical-energy-other-forms) — Utilization of Electrical Energy, chapter 1, asked 2×
- [Protection for alternator and step-up transformer](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Amega-project-protection) — Power Plant Design, chapter 4